Device for tuning a VCO to the carrier frequency of each radar pulse

ABSTRACT

PCT No. PCT/IT85/00014 Sec. 371 Date May 12, 1986 Sec. 102(e) Date May 12, 1986 PCT Filed Jul. 3, 1985 PCT Pub. No. WO86/00768 PCT Pub. Date Jan. 30, 1986.An electronic device for rapidly tuning a voltage controlled oscillator to the radar carrier frequency of each of the individual radar pulses. A frequency discriminator (1) of the quadricorrelator type is supplied with the incoming pulses and with the output of the VCO (5) to provide the control voltage to the VCO (5) via amplifying stages (2), (4) and a sample-and-hold circuit (3) that maintains the tuned frequency for time durations greater than 100 usec. The operating frequency range of the device is between 6GHz-18GHz.

BACKGROUND OF THE INVENTION

The present invention relates to an electronic device which can tune avoltage controlled oscillator (VCO) carrier frequency, pulse to pulse,to the carrier frequency of a radar pulse, operating over bands beyondone octave.

Such device can compensate for frequency errors up to ±500 MHz in a timeperiod shorter than 100 μsec. As the device is equipped with a memorycircuit, signal lock may be maintained for the time sufficient to allowto any type of intentional interference.

The invention belongs to the radar field, and more exactly, to the fieldof "frequency tuning devices". It finds specific application in thefield of Electronic Counter Measures. Here, the function of such devicesis to effect in the shortest possible time, VCO tuning to the radarpulse carrier frequency. This way, further modulation of the VCO aroundsuch carrier, by means of noise or interference, may mask the radarreturn echo so as to deprive it of presence information in the case ofsurveillance and of tracking radars. At present, tuning problems aresolved, in general, through devices such as:

PLL (Phase locked loop)

AFC (Automatic Frequency Control)

The PLL uses a phase detector as a comparator between input signal andVCO phase. This phase difference at the phase detector (PD) output isconverted into a difference of potential, which applied to the VCO,changes its frequency in a way to reduce the initial phase error.

The AFC is based upon a frequency discriminator which may be of twotypes:

Intermediate frequency discriminator

Interferometric frequency discriminator

In the case of the IF discriminator, two resonant circuits tuned tofrequencies symmetrical to a prefixed reference frequency are used;resonance curves, one of which inverted, (if the detected signaldifference is calculated) give way to the typical S voltage-frequencycurve of the discriminator, the null point of which coincides with theprefixed reference frequency.

In the case of the interferometric discriminator the input signal issplit into two branches, one of which reaches a phase detector directly,while the other one reaches destination via a delay line.

The phase detector output is a voltage proportional to the phase betweenthe two signals. This phase difference, in turn, is proportional to theinput signal frequency.

The voltage proportional to the frequency difference, sent to the VCO,is such that through the AFC loop, the frequency received and that ofthe VCO are made to coincide.

In a tuning system using the PLL principle, there is a tight relationbetween lock-in band and loop band.

This means that when a lock-in band of a few hundred MHz is required,one PLL alone does not solve the problem adequately for obvious physicalreasons relating to component bandwidth.

In a tuning system using the AFC principle, lock in time is related tothe loop bandwidth, and the maximum Δf for which the loop can lock isexclusively set by the frequency discriminator characteristics. It is awell known fact that an IF type frequency discriminator has a limitedlock in range (tens of MHz) while an interferometric discriminator,which gives way to wider lock in ranges, has the drawback of largedeviations from linearity, as well as noise and dynamics problems.Furthermore lock-in times are longer and the loop is not suitable forultrarapid tuning.

In any case these discriminators, working at fixed referencefrequencies, do not give fast lock in because the frequency measurement,onto which lock in has to take place, and that of VCO, must be made onthe same discriminating element at different times.

SUMMARY OF THE INVENTION

Objects of the present invention are to:

1.provide a discriminator/tuning circuit which can tune, pulse to pulse,the VCO frequency line to the radar pulse carrier frequency;

2. correct errors greater than ±500 MHz within a time period not greaterthan 100 nsec;

3. keep radar signal lock for the time sufficient to enable any type ofintentional interference, such as >100 μsec, by means of a memorycircuit inside the loop.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate further discussion of the present invention, the followingdrawings are provided in which:

FIG. 1 shows a block diagram of one embodiment of the invention; and

FIG. 2 shows a block diagram of one embodiment of the internal structureof the frequency discriminator of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The figures may be described in further detail as follows. FIG. 1 showsa block diagram of the device, where the elements listed below have theindicated reference numerals:

a frequency discriminator (1)

two video amplifier stages (2) (4)

a Sample & Hold sampling and memorizing circuit (3)

a Voltage Controlled Oscillator (5).

FIG. 2 shows the frequency discriminator circuit block diagram shown as(1) in FIG. 1. It is made up of:

three mixers (6) (8)

a differentiator circuit (7)

a 90° hybrid (9)

two RF amplifiers (10).

As can be seen from the figures, the radar signal, arriving from apossible threat (missile en route), arrives at the frequencydiscriminator (1) and a d.c. term appears at its output which isproportional to the input frequency difference (coming one from theVCO/5 performing voltage to frequency conversion, the other from theradar) with their sign, and a frequency term twice that of the frequencydifference, which varies with a cosine law.

The term useful for the loop is the d.c. one, corresponding to the radarpulse.

The two video amplifier stages (2) and (4), doubled for dynamic reasons,serve to fix the correct gain value of the loop to enable lock-in from aprefixed initial Δf with the accuracy desired.

A S/H circuit (Sample and Hold or memory circuit (3)) maintains, forsome time, the VCO line (5) onto that of the signal once the tuningphase is over.

VCO (5) varies its frequency rapidly (30 nsec) as the output voltagefrom the discriminator (1) changes.

With reference to FIG. 2 and calling f₅ and f_(VCO), respectively, theinput signal frequency (belonging to a given threat such as a missile enroute) and that of the VCO, the following signals will be present atpoints A, B and C: ##EQU1##

In (1) the sum frequency term has been neglected as such term eitherfalls outside the operational band or is about 20 dB below thedifference frequency term.

(Such assumption holds true for all the sum frequency terms.) For thesame reason, low pass filters at the output of (6) have not been used.##EQU2##

It is worth noting that the differentiator makes use of an RC structureusing a very low loss, high precision capacitor chip mounted onto a 50ohm line manufactured on an alumina substrate (Al₂ O₃).

As the capacitor is precision made so that the electrodes areeffectively two parallel planes having uniform thickness and dimensionscontained within the defined tolerances, precisions on the capacityvalue of the order of ±0.01 pF are obtained.

The manufacturing tolerances and the very low loss dielectric are suchthat inductive and resonant phenomena are negligible, so that thecapacitor manufactured continues to behave as an ideal capacitor even atextremely high frequencies.

The fixing of the device into the substrate is achieved through thermalcompression taking care to minimize terminal length to reduce allparasitic effects. ##EQU3##

From the last relation it can be seen that the output of the FIG. 2circuit produces a continuous term which is proportional to thefrequency difference in sign and to a frequency term which depends ontwice the difference frequency, which varies with a cosine profile. Thed.c. term is the useful one in terms of loop performance, while thecosine term, for high Δf durations, is averaged or filtered by the loopand for Δf_(S) within the loop band, it modulates the VCO in anirrelevant manner.

As in an AFC loop the following holds true: ##EQU4## where:

ω.sub.η is the closed loop band and t is the lock-in time. To achieveextremely short lock-in times it is necessary to insert into the loop afilter such that through a suitable loop gain, the closed loop gain fitsrelation (5) with t<100 nsec.

To this end the sample/hold (3) time constant is exploited, whereby itis dimensioned to achieve the desired closed loop cutoff frequency.

The video amplifiers, twin for dynamics reasons, are such that withdiscriminator kd and VCO K_(V) the correct open loop gain is achieved toobtain the desired performance.

The device of this invention utilizes, in an original manner, a quadricorrelator as a frequency discriminator in a RF tuning loop. Normal RFmixers are used, obtainable on the market and no recourse is made todelay lines as in present interferometric discriminators. This allows avery wide operational bandwidth of the device, which is limitedexclusively by the present microwave components. A peculiarity of thisdiscriminator is the characteristic voltage frequency S curve which doesnot have its null at a prefixed frequency, but, rather, presents itsnull always correspondingly to the frequency of the subject signal overthe whole working band.

The assembly achieves in simple manner absolutely outstandingperformance, which similar configurations have never achieved:

    ______________________________________                                        loop operational bandwidth                                                                       >1 octave eg 6.5:18 GHz                                    lock in band       >±500 MHz                                               lock in time       <100 nsec                                                  frequency error starting from                                                                    <5 MHz                                                     maximum deviation                                                             AFC loop bandwidth >2.5 MHz                                                   ______________________________________                                    

The device of the present invention, is particularly useful for thoseoperational situations where fast switching of a radar signal isrequired. Furthermore, due to its high performance, including very fastlock in time, it is suitable for inclusion in systems for agile andcoded radars interference which are to date immune to electronic warfareequipment.

We claim:
 1. A device for rapidly tuning the frequency of a voltagecontrolled oscillator to the carrier frequency of each pulse of a radar,comprising:a frequency discriminator having two inputs and adapted toreceive radar pulses directly to one of the two inputs; a firstamplifier coupled to the output of the frequency discriminator; asample-and-hold circuit coupled to the output of the first amplifier; asecond amplifier coupled to the output of the sample-and-hold circuit;and a voltage controlled oscillator coupled to the output of the secondamplifier, the output of which voltage controlled oscillator is in turncoupled to the second input of the frequency discriminator.
 2. Thedevice of claim 1 in which the frequency discriminator is comprised ofat least two input mixers, a 90° hybrid coupled to one input of each ofthe input mixers, a differentiator coupled to the output of one of themixers, at least two amplifiers, and an output mixer, such that a directcurrent term is generated at the output of the output mixer which isproportional to the difference frequency in sign and to a frequency termwhich is twice the frequency difference and varies as the cosine of theinput.
 3. The device of claim 2 in which the frequency discriminatoroperates as a quadricorrelator in an R.F. tuning loop.
 4. The device ofclaim 3 having an operating frequency range of 6GHZ-18GHz.
 5. The deviceof claim 4 which maintains the tuned frequency of the voltage controlledoscillator for time durations greater than 100 microseconds.